1. Field of the Invention:
The present invention relates to a process for treating wastes by liquid phase hydrogenation in the presence of a catalyst or additive of the liquid products, as well as low temperature tar and low temperature coke, which are obtained by the low temperature carbonization of wastes such as plastic waste, hydrocarbon-loaded residues, contaminated soil, biomasses, sludge and the like;
2. Discussion of the Background:
The low temperature pyrolysis of household garbage, industrial and commercial waste, and special waste in a suitable reactor, e.g., a rotary kiln is known (cf. DE-PS 29 17 293, EP 0 111 081). Depending on the operating parameters of the plant and the materials that are used, the products of pyrolysis are low temperature gas, coke, oil, or tar.
Depending on the kind and origin of the waste products to be carbonized, the low temperature products contain a wide variety of pollutants.
In addition to the so-called system-inherent pollutants, which are already contained in the waste products prior to carbonization, special attention must be paid to the process specific pollutants. Under the reducing conditions of the low temperature carbonization process, in addition to the major components (hydrogen, carbon monoxide, carbon dioxide, methane and water), hydrogen compounds such as ammonia, hydrogen sulfide, hydrogen cyanide, hydrogen chloride, and hydrogen fluoride as well as among others NO.sub.x and sulfur dioxide occur. The gaseous pollutants can be trapped by adding basic slagging materials such as lime, and they then remain primarily in the low temperature coke (cf. EP 0 111 081).
Depending on the operating parameters of the low temperature carbonization process and the substances that are used, polycyclic aromatic hydrocarbons, in particular polychlorinated dibenzodioxines and polychlorinated dibenzofurans, can also occur. When the low temperature gases cool, these substances condense with other formed aromatic and phenolic hydrocarbons. In order to remove the sulfur vapors from the pollutants thus-obtained and to make them useful for further applications, usually one or more process steps are added to the pyrolysis process for treating the low temperature products.
A portion of the low temperature gas is usually used as the heating gas in the low temperature carbonization process. For this purpose the low temperature gas is first treated in a gas converter in such a manner that it is partially oxidized and dissociated above a red hot bed of coals at 1200.degree. C. (cf. DE-OS 33 17 977). The pyrolysis gases, which are combusted to flue gases and which are not necessary for heating purposes, are purified and released by means of a chimney (cf. EP-0 111 081). In another process the pyrolysis follows a cracking reactor which is heated by means of electric resistance heating (cf. DE-PS 29 35 669). In DE-OS 34 12 582, a gas converter following the low temperature step is described.
Another process for treating low temperature products arising from the pyrolysis of waste is treatment in a melting chamber in which high temperature combustion occurs (cf. DE-OS 36 05 693).
According to DE-PS 29 17 293, low temperature gases, which are obtained from the pyrolysis of old tires, old cables, or plastics, are subjected to a further heat treatment at from 900.degree. K. to 1200.degree. K. in a tube bundle reactor. In the subsequent condensation step, an aromatic-rich oil is obtained that can be used as chemical and mineral oil raw material.
Depending on the kind and composition of the waste material to be carbonized, the pyrolysis process is subject to modified demands. The efficiency of these processes must always be regarded in relation to their environmental compatibility. Minimizing the release of heavy metals, acidic gases and polycyclic chlorinated hydrocarbons requires a high engineering cost. Especially with heterogeneous waste, the products of pyrolysis can usually be used only for energy. The recovery of valuable chemicals has come under consideration from an economical point of view only for specific kinds of special waste such as old tires (cf. DE-OS 24 62 495).
Thus, there remains a need for an inexpensive process for treating wastes which does not result in the release of pollutants such as polycyclic hydrocarbons or chlorinated hydrocarbons into the environment and allows the recovery of usable products.